The danger of infection from accidental contact with the tip of a used hypodermic needle has long been recognized and is well documented. In most procedures, the greatest avoidable risk of accidental needle puncture, also referred to as a "needlestick", occurs during handling of the used needle, when an operator generally inserts the used needle into a protective sheath for disposal. This action usually requires the operator to move the hand which holds the sheath toward the pointed tip of the needle, and any inaccuracy in this operation raises the possibility of a puncture. The risk of this is greatly increased if the operator is working under stress, such as time pressure or fatigue, or is handicapped by marginal eyesight or unsteady hands.
The problem of accidental needlesticks has recently engaged the attention of the United States Congress. The "Health Care Worker Needlestick Prevention Act" was recently introduced in both the House of Representatives and the Senate. The Act, if passed, would require the Secretary of Labor, acting through the Department of Labor's Occupational Safety and Health Administration, to amend OSHA's blood borne pathogens standard to require that, among other things, employers utilize "engineered sharps injury protections." The Act generally defines "engineered sharps injury protections" as physical attributes that effectively reduce the risk of a needlestick injury and that are built into needle devices or other sharp, non-needle devices that are encountered in a health care setting.
A number of such "engineered sharps injury protections" have been developed for needle devices to deal with the general problem of needlesticks. One such device is disclosed in U.S. Pat. No. 5,322,517 to Sircom et al., entitled "Disposable Automatic Hypodermic Needle Guard" ("the Sircom Patent"). The Sircom Patent discloses several embodiments for a protecting device for covering the tip of a needle. The protecting device takes the form of a protective guard which is initially stored on the barrel of the needle and remote from its tip, and which is displaceable to a position enclosing the needle tip where it automatically locks in this position, serving as a protecting means for the needle tip. In the embodiment shown in FIGS. 35 through 39, a locking device includes a pivoting or lever arm or lever 101 of stiff material, formed in the general shape of a broad U of unequal proportions, with the longer vertical leg, referred to as the locking plate 102, provided with a hole 103 of slightly larger diameter than that of the needle shaft 1 sufficient to allow the locking plate 102 to "cant" on the needle 1, as shown in FIG. 39 of the Sircom Patent.
A helical compression spring 104 is contained in a largely compressed state in longitudinal cavity 105 within the guard body 106, with its free end 125 pressing against the locking plate 102 of the lever 101 and urging the level 101 axially against the sloping internal face 107 of body 106, making contact at pivot point 108. The turning moment of the spring force tends to rotate the level 101 in a counter-clockwise direction about the pivot point 108, but the shorter vertical leg 109 of the lever 101, referred to as the latching or locking leg, prevents rotation by making contact with the needle shaft 1 at point 110.
The slope of the face 107 in relation to the geometry of the lever 101 is chosen to develop a component of reaction force that acts at the pivot point 108 in a downward direction against the lever 101, equal and opposite to the upward reaction force against the lever at the point 110. This balancing of vertical forces against the lever 101 substantially removes any radial force between the needle shaft 1 and the walls of the hole 103 into lever 101, thereby reducing axial frictional drag between the needle shaft 1 and the lever 101 when moving the guard device axially along the needle shaft.
When the needle shaft 1 is withdrawn into the protective guard and passes the point of contact with the sensing end of the locking leg 109 of the lever or pivot arm 101, the lever 101 rotates in a counter-clockwise direction about the pivot point 108 under the urging of the spring 104, which rotates the locking plate 102 until further rotation is prevented by the axial misalignment of the hole 103 and the needle shaft 1.
A problem with the protective guard described in the Sircom Patent is its complexity. For example, the protective guard requires a critical geometry to ensure that the lever 101 locks on the needle shaft I when the needle shaft 1 is retracted into the protective guard. In establishing this critical geometry, care must be taken to ensure a particular relationship between the thickness 111 of the latch 101, the distance 112 between the pivot point 108 and the center of the needle shaft 1, the diameter of the hole 103, the diameter of the needle shaft 1, and the coefficient of friction between the latch 101 and the needle shaft 1.
Another problem with the protective guard described in the Sircom Patent is that the locking leg 109 of the lever 101 does not provide a mechanism whereby a wire guide can pass therethrough after the needle shaft 1 has been withdrawn into the protective guard. It is desirable to enable a wire guide to pass through the protective guard after the needle tip has been retracted into the protective guard so that the wire guide can facilitate the advancement of a catheter into the blood vessel of a patient.